In the case of optically readable circular disk form recording media such as optical disks, optical-magnetic disks and the like, not only media for playback use only, but also media that allow the rewriting of recorded information, have become widely popular. Such recording media are manufactured by bonding substrates to each other in order to protect the recording surfaces formed on the substrates, or in order to realize high-density recording by the multi-layer formation of recording surfaces.
For example, the manufacture of such recording media is accomplished by the procedure shown in FIG. 12. Specifically, two substrates made of a polycarbonate are injection-molded (1001), and a metal film is formed on these substrates by sputtering in a sputtering chamber (1002). Then, the joining surfaces of the two substrates are coated by spin coating with an ultraviolet curable adhesive agent (1003). The pair of substrates thus coated with an adhesive agent is inserted into a vacuum chamber, and the adhesive-agent surfaces are bonded to each other in a vacuum (1004). The substrates thus bonded together are removed from the vacuum chamber and exposed to atmospheric pressure, and the adhesive agent is cured by irradiation with ultraviolet light (1005). As a result, the two substrates are firmly bonded together, and the disk is completed. In particular, a technique such as that disclosed in Patent Reference 1 has been proposed in the past as a disk manufacturing apparatus commonly using a rotating turntable to convey the substrates.
If warping (tilting) occurs in a disk manufactured as described above, there is a possibility that the laser light used to read and write information will not accurately reach the specified position on the recording surface when the disk is irradiated with this laser light. Accordingly, in such optical disks, the exclusion of such warping is important for ensuring stable quality of the product.
In order to deal with this problem, a technique in which so-called annealing that eliminates or relaxes residual stress in the material by heat-treating the substrates before or after bonding has been proposed as described in Patent Reference 2 and Patent Reference 3.    Patent Reference 1: Japanese Patent Application Laid-Open No. 2000-348389    Patent Reference 2: Japanese Patent Application Laid-Open No. 2001-14736    Patent Reference 3: Japanese Patent Application Laid-Open No. 2002-92969    Patent Reference 4: Japanese Patent Application Laid-Open No. 10-255340